FIG. 1 shows a typical laser printer 10 including a movable photoreceptor 12, typically a revolving photosensitive drum or cylinder. This drum 12 is made out of a highly photoconductive material that is discharged by light photons. Initially, the photoreceptor drum 12 is given a total positive charge by a charging electrode 14, typically a wire or roller having a current running through it. As the drum 12 revolves, the printer 10 uses a laser unit 16 (such as a laser diode) to shine a laser beam 18 across the surface of the drum 12 to discharge certain points. In this manner, the laser beam 18 “draws” the text and images to be printed as a pattern of electrical discharges (an electrostatic image) on the drum 12. If the laser beam 18 is modulated, resulting variations in charge on the drum 12 will ultimately be translated to proportionate amounts of toner deposited on a sheet of paper 21.
After the laser beam 18 scans the desired electrostatic pattern on the drum 12, the printer 10 uses a toner roller 20 to coat the drum 12 with positively charged toner powder. Since the toner has a positive charge, it clings to the negative discharged areas of the drum 12 that have been scanned by the laser beam, but the toner does not cling to the positively charged “background” of the drum. With the toner pattern affixed to the drum 12, the drum rolls over the sheet of paper 21 traveling below it. Before the paper 21 travels under the drum 12, the paper is given a negative charge that is stronger than the negative charge of the electrostatic image on the drum 12 so that the paper pulls the toner powder away from the drum 12. Finally, the printer 10 passes the paper 21 through a fuser 24, which is typically a pair of heated rollers and as the paper 21 passes through the fuser 24, the loose toner powder on the paper melts, fusing with the fibers in the paper and forming a permanent image on the paper. After the toner on the drum 12 is transferred to the paper 21, the drum surface passes a discharge lamp 22 that generates a bright light that exposes the entire width of the photoreceptor surface of the drum 12, erasing the electrostatic image. The drum surface then passes the charging electrode 14, which reapplies a positive charge to the surface of the drum 12 in anticipation of the laser beam 18 scanning the next image to be printed onto the drum.
FIG. 2 is a functional top view of the printer 10 of FIG. 1 showing a number of dark areas 200 on the drum 12 that represent the negatively discharged regions, which may be alternately referred to as dots, created by the scanning laser beam 18. FIG. 3 shows a perspective view of portions of the laser printer 10 in FIG. 1 better showing the scanning of the laser beam 18 in a horizontal direction across the drum 12 as indicated by an arrow 300. Image processing circuitry (not shown) controls the laser unit 16 to modulate the laser beam 18 as the beam scans across the drum 12 in the horizontal direction 300 one line at a time. The image processing circuitry controls the laser unit 16 to turn ON and emit a pulse of light for every dot to be formed in a given horizontal line and to turn OFF where no dots are to be formed in the line. Pixel data stored in the image processing circuitry stores data for an array of pixel corresponding to the image being printed, with data for each pixel in the array corresponding to a discharge region or dot to be formed on the surface of the drum 12.
In scanning the laser beam 18 across the drum 12, the laser unit 16 does not actually move the laser beam 18 itself but instead bounces the laser beam 18 off of a movable mirror 26, such as a rotating mirror. As the mirror 26 moves, it reflects the laser beam 18 through a series of lenses (not shown) and onto the drum 12. These lenses change characteristics of the laser beam 18 to compensate for image distortion that would otherwise be caused by the varying distance between the mirror 26 and points along the drum 12. Also note that although FIGS. 2 and 3 illustrate the laser beam 18 scanning across the drum 12 in the horizontal directional, the actual scans of the laser beam are angled or “slanted” due to the movement of the drum 12 as the beam scans across the surface of the drum, as will be appreciated by those skilled in the art. For example, as the beam 18 scans from left-to-right across the surface of the drum 12, the drum is rotating in the direction indicated by an arrow 302 in FIG. 3. As a result, without any compensation scan lines of the laser beam 18 from left-to-right would be angled upward from left-to-right since the surface is moving as the beam 18 scans across that surface. The amount of slanting of the scan lines traversed by the laser beam 18 is quite small, and all scan lines have the same amount of slant. As a result, conventional printers 10 do not typically compensate for the slanting of the scan lines traversed by the laser beam 18 since the slant does not cause significant visually noticeable errors in printed pages.
Oscillating mirrors in the form of micro-electromechanical systems (MEMS) devices have become increasingly popular in various types of electronic systems. Such MEMS devices are fabricated using techniques generally used in fabricating microelectronic devices and are thus small, inexpensive, and can easily be integrated with associated electronic circuitry. As a result, the use of an oscillating mirror in place of the rotating mirror 26 in the printer 10 is desirable. A typical oscillating mirror includes a reflective portion that rotates or oscillates about pivot arms. Electronic circuitry develops electromagnetic fields that control the oscillation of the reflective portion. When an oscillating mirror is used in place of the rotating mirror 26, during a portion of the oscillation period of the mirror the laser beam 18 is reflected off the reflective portion to thereby scan the beam across the surface of the drum 12 either from left-to-right or from right-to-left. The laser beam 18 is then turned OFF as the oscillating mirror rotates to a maximum position about the pivot arms and then rotates back to a starting position, at which point the laser beam is again turned ON and modulated to form the next row of dots on the surface of the drum 12.
To use an oscillating mirror in the printer 10, the frequency of oscillation of the mirror about the pivot arms and the speed of rotation of the drum 12 must be constant for proper operation. This is true because for a given speed of rotation of the drum 12, the oscillation frequency of the mirror must scan the laser beam 18 from left-to-right across the surface of the drum and then the mirror must rotate back to an initial position such that the laser beam is ready to scan the next row of dots. After scanning a given row of dots, the time it takes the mirror to rotate back to the initial position and be positioned to scan the laser beam 18 to form the next row of dots is known as the “turnaround time” of the mirror.
Rather than being constant among mirrors, however, the oscillation frequency varies slightly from mirror to mirror. When the oscillation frequency is faster than desired, consecutive scan lines of the laser beam 18 overlap adjacent rows of dots being formed on the surface of the drum 12. In this situation, the laser beam 18 begins scanning the next row of dots prior to the drum 12 having rotated as far as desired and as far as it would have for the desired oscillation frequency of the mirror. To compensate for such variations in oscillation frequency, intensity data associated with dots being formed may be scaled to achieve the desired discharge for each dot. Since the overlap is constant for dots being formed in a given row, the intensity data of dots in adjacent columns of dots can be weighted to compensate for the overlapping scan lines. Although not discussed in detail herein, similar problems result when the oscillation frequency of the mirror is slower than desired, as will be appreciated by those skilled in the art. Also, adjusting the speed of rotation of the drum 12 as the oscillation frequency of the mirror varies is possible but is undesirable because this may adversely affect the operation of other components in the printer 10, such as the fuser 24 which may not properly fuse toner with the paper 21 if the rotational speed of the drum increases.
While the use of oscillating mirrors in laser printers 10 has advantages, such as reduced cost, such mirrors have not been commercially implemented for a number of reasons. For example, note that the operation of the oscillating mirror described above is “unidirectional” since the mirror scans the laser beam 18 only from either left-to-right or right-to-left across the surface of the drum 12. This means only half the oscillation period of the mirror is used for scanning the laser beam 18, while the other half effectively functions as the “turnaround” time of the mirror.
Because the turnaround time is approximately half the oscillation period of an oscillating mirror, the turnaround time for oscillating mirrors is relatively large when compared to the rotating mirror 26. As a result, there is less time available for the laser beam 18 to scan the surface of the drum 12 and form the required dots. This is true because for a given speed of the drum 12 there is a set time between the scan of the beam 18 to form a first row of dots and the scan of the beam to form the next row of dots. The time available for the laser beam 18 to scan the surface and form dots is equal to the set time minus the turnaround time, and the longer turnaround time for the oscillating mirror makes this available time smaller and thus necessitates the beam move faster to form the required dots within this smaller time. The faster scanning of the laser beam 18 means the power of the laser beam 18 must be increased to sufficiently discharge regions on the surface of the drum to form the required dots. Higher power lasers for generating such a laser beam 18 are more expensive and thus undesirably increase the overall cost of the printer 10.
To alleviate the adverse affects of the increased turnaround time of oscillating mirrors, the laser beam 18 ideally could be reflected off the mirror 26 to “bidirectionally” scan the laser beam across the surface of the drum 12 from left-to-right and from right-to-left. Such bidirectional scanning is not used in printers 10 containing oscillating mirrors, however, because bidirectional scanning does not allow scaling of intensity data as required to compensate for variations in the oscillation frequency of the mirror as described above. This is true because bidirectional scanning of the laser beam 18 results in scan lines across the surface of the drum 12 that form a “zig-zag” pattern 304 on the surface as depicted in FIG. 3. More specifically, scan lines 306 of the laser beam 18 from left-to-right slant upward from left-to-right since the surface is moving as the beam 18 scans across the surface, as previously discussed. Conversely, if the beam 18 is being scanned bidirectionally then scan lines 308 of the laser beam 18 from right-to-left slant upward from right-to-left. When combined with variations in the oscillation frequency of the oscillating mirror, this zig-zag pattern 304 of the scan lines results in a variable overlap of dots in adjacent rows of dots being formed and thus does not allow for scaling of intensity data as described above for the constant overlap that is present in unidirectional scanning systems.